1. Field of the Invention
The present invention relates generally to solder masks and use thereof in packaging semiconductor devices and, more specifically, to a method for encapsulating portions of a semiconductor device package using a solder mask as a mold for the encapsulant material.
2. State of the Art
As the dimensions of electronic devices are ever decreasing, the sizes of the structures used to package the microprocessors, memory devices, other semiconductor devices, and other electronic componentry must also become more compact.
One approach to reducing the size of semiconductor device assemblies is to minimize the profiles of the semiconductor devices, as well as the connectors and the electronic components to which the semiconductor devices are electrically connected, as well as to minimize the overall profiles of such assemblies. One type of packaging technology that has been developed to save space in this manner is the so-called “chip-scale package” (CSP).
An example of a CSP designed to save space is a board-over-chip (BOC) package. A typical BOC package comprises a carrier substrate that is configured to be secured over the active surface of a semiconductor die, wherein bond pads of the semiconductor die are exposed through an opening formed through the carrier substrate. The bond pads on the semiconductor die are connected to conductive elements on the carrier substrate using a step where wire bonds are formed and electrically connect the bonds pads to the conductive elements.
Following wire bonding, it is desirable to encapsulate the wire bonds between the semiconductor die and the carrier substrate. Encapsulation serves a variety of functions, including sealing the encapsulated surfaces from moisture and contamination and protecting the wire bonds and other components from corrosion and mechanical shock.
Encapsulants may be deposited from the top of the carrier substrate to encapsulate the semiconductor die and wire bonds. The material used for the encapsulant typically comprises a flowable, dielectric material. Alternatively, a glob-top or other encapsulant may be formed over the wire bonds for protection. Glob-top structures use a high viscosity encapsulant, typically a silicone or an epoxy, such that the encapsulating material may be applied to a substantially planar surface without being laterally confined. However, the height of the resulting glob-top structure may be higher than is required to properly encapsulate the wire bonds and may interfere with subsequent packaging steps.
After encapsulation, a solder stencil or solder mask may be placed or formed on the surface of the carrier substrate. Solder stencils and solder masks typically include a number of openings in which solder balls may be placed or formed.
Conventional solder paste stencils and solder ball placement stencils are substantially planar metal structures that are aligned with and secured to a bond pad-bearing surface of a semiconductor device or a terminal-bearing surface of a carrier substrate, such as a printed circuit board, on which solder balls are to be formed. Apertures that have been formed through the stencil are aligned with corresponding bond pads or terminals. Such conventional solder stencils are designed to resist the adherence of solder and, thus, of the formed solder balls thereto. Once such a solder stencil has been secured to a semiconductor device or a carrier substrate, solder may be introduced onto the solder stencil, for example, by at least partially immersing the component or an assembly that includes the component in a solder bath to form solder balls on bond pads or terminals that are exposed through apertures of the solder stencil. When solder balls have been formed, a conventional metal solder stencil is typically removed from the component from which the solder balls protrude, then cleaned, and reused.
State-of-the-art solder masks are typically single-use structures that are formed directly on the component on which solder balls are to be formed. These single-use solder masks may be formed from a photoimageable material that, when cured, will withstand the conditions to which such solder masks will be exposed, such as the typically high temperatures of molten solder. Solder balls may be formed by employing the same types of techniques, as described above, that are used with conventional, metal solder masks. Once the solder balls are formed, if the single-use solder mask was formed from a dielectric material and the solder balls protrude a sufficient distance therefrom, the single-use solder mask may remain in place on the component. Alternatively, the solder mask may be removed from the component, such as by use of suitable photoresist stripping agents, to further expose the solder balls.
The solder mask prevents bridging of the solder material and shorting between the solder balls in the completed package. The presence of a glob-top structure may, however, make it difficult to place the solder mask over the carrier substrate, particularly if the glob-top material has moved too far laterally.
Accordingly, there is a need for a solder mask that may be positioned on a carrier substrate of a semiconductor device assembly prior to encapsulation of bond wires and which may remain in place as wire bonding operations are being conducted, as welt as for assemblies and packages including such solder masks and methods for forming and using such solder masks.